What is OTT?

OTT in plain English

A normal compressor only ever turns things down. When a sound goes over its threshold it gets quieter, and anything under the threshold is left alone. OTT does that too, but it also works from the other side: when a sound drops below a second, lower threshold, OTT turns it up. Loud moments are pulled down and quiet moments are pushed up, so both ends of the dynamic range move towards the middle.

It does this separately in three frequency bands. The lows, the mids and the highs each get their own pair of compressors, so a loud kick drum turns down the low band without pulling the hi-hats down with it. That makes OTT a multiband compressor, and the multiband compression lesson covers the idea in general.

The name comes from Ableton Live. Its Multiband Dynamics device shipped with a preset called OTT, short for “over the top”, which pushed all three bands hard in both directions. Xfer Records later released OTT as a free plug-in that recreates the preset with a few simple controls, and when people say “OTT” today they usually mean that plug-in.

Upward and downward compression

Downward compression is the kind you already know. Say the upper threshold is −28 dBFS with an 8:1 ratio, which is how the lab is set up. A band at −20 dB is 8 dB over, so it comes out 1 dB over, at −27 dB: it's been turned down by 7 dB.

Upward compression is the mirror image. Below the lower threshold, the distance under the line gets divided by the ratio instead of the distance over it. With the lab's lower threshold at −36 dB and a 4:1 ratio, a quiet passage at −52 dB is 16 dB under, so it comes out 4 dB under, at −40 dB. It has been turned up by 12 dB, and nothing about the loud parts changed.

Turning up quiet sound needs a limit, because the quietest thing in any recording is noise. The lab stops lifting at 18 dB, so a band in near silence comes up by 18 dB and no more. That's still enough to bring a noise floor at −80 dB up to −62 dB, which is why OTT has a reputation for making hiss audible.

With both halves working, each band ends up squeezed into the narrow space between the two thresholds. On the drum loop in the lab, the gap between the loud and quiet moments shrinks from about 45 dB to 28 dB. You can see the shape on the transfer curve under the loop: the line bends up on the left, runs straight through the middle and bends down on the right.

Why three bands?

The lab splits at 88 Hz and 2.5 kHz, like OTT. Everything below 88 Hz is the low band, which on most sounds means sub-bass and the weight of a kick. The mid band holds the body of nearly everything. Above 2.5 kHz is the high band, where cymbals and consonants live, along with the brightness of a synth.

Because each band is compressed on its own, OTT changes the tone of a sound as well as its dynamics. The upward half lifts the quietest band the most, and in most sounds that's the highs. On the chords loop in the lab, the high band sits far below the mids and OTT raises it by about 18 dB on average, so a soft, dark pad comes out bright and airy. A dull synth patch gets the same treatment, which is a big part of why sound designers reach for it.

On a vocal or a full mix the same thing works against you, because the quietest high-frequency material there is often sibilance, breaths and reverb tails. OTT lifts those just as willingly.

The crossovers in the lab are Linkwitz-Riley filters, the usual type for splitting bands, because the bands add back up to a flat frequency response when nothing is being compressed. Set Depth to 0% and the output sounds the same as the input; the only change is a small phase shift around the two crossovers, which you can't hear on its own.

Reading the lab

The lab is a model of OTT: a three-band upward and downward compressor with the same set of controls, using its own thresholds and timings, so you can see what each part of the process does. The big screen shows the whole loop: the pale shape is the original and the pink shape is what comes out. Where the pink fills in the space between hits, quiet parts have been turned up.

Under it, the band lanes show each band's compressor across the loop. A line above the middle means that band is being turned up at that moment and a line below means it's being turned down, scaled to 18 dB either way. The meters beside the loop show the same thing live while the loop plays.

The transfer curve plots band level in against level out, with the lower threshold in teal and the upper one in pink, and the coloured dots are the three bands riding up and down the curve as the loop plays. Next to it, the band screen draws the three crossover filters, each raised or lowered by how much that band changes on average. It's the overall tone change, and on most sounds the high band climbs the furthest.

The panel on the right describes what your settings are doing, with numbers measured on the loop. Press A/B to switch the processing off; the lab turns the original up or down to match the processed version's loudness, so you hear only the change in shape and tone. You can also load your own sound or record eight seconds with your microphone, and it never leaves your device.

The controls

OTT has very few controls. The preset already decided the thresholds, ratios and crossover frequencies, so none of them has a knob. You get six main controls and a gain slider for each band.

Depth

Depth is a dry/wet blend for the whole effect. At 100% you hear the processed bands only; at 0% you hear the original. In between, each band is a mix of its compressed and uncompressed self, the same idea as parallel compression. At 30%, a band the compressors would lift by 12 dB comes up by about 5.5 dB, while a band they'd cut by 12 dB only comes down about 2.2 dB, because the untouched copy is always there underneath.

100% is the effect at full strength, and that's the setting that gave OTT its reputation. Most of the time you'll want far less. On a bus or a whole mix, try 10 to 30%; on a synth or a sound effect, 30 to 60% is a good place to start.

Time

Time scales the attack and release of every band together. Higher is slower and lower is faster. In the lab, 100% gives the low band a 2 ms attack and a 120 ms release, and the mid and high bands are faster again.

Bring Time down and the compressors follow the sound more closely. The effect gets more extreme, because the gain now moves inside each note as well as between notes, and it adds grit. At 10% the low band's release in the lab is about 16 ms, and a 55 Hz bass note takes about 18 ms to complete one cycle, so the gain starts tracking the waveform itself. That's distortion, and on a growl bass it might be exactly what you want. On a mix, keep Time high.

Upward and Downward

These two knobs set how strongly each half works, from 0 to 100%. With Upward at 0% you have a plain three-band downward compressor. With Downward at 0% you have pure upward compression, which keeps the loud hits as they were and fills in everything around them.

Once you know what each half sounds like, these are the most useful controls on OTT. If the processing makes a vocal hiss, Upward is the knob to turn down. If drums lose their snap, it's Downward. Try each one at 0% in the lab, then bring it back up until just before the problem returns.

In gain and Out gain

OTT's thresholds don't move, so In gain is how you decide how hard it works. Turn the input up and every band sits higher, so more of it goes over the upper threshold and gets turned down, and less of it falls under the lower one. Turn the input down and the balance tips towards upward compression.

Out gain is plain volume at the end, the same job as makeup gain on a normal compressor. With both halves at full depth the lab's drum loop comes out about 7.5 dB quieter than it went in, and +8 dB of Out gain brings it back to the same loudness.

Low, Mid and High

OTT has a gain control for each band, marked L, M and H on the plug-in. The lab's band controls turn each band up or down after the compression, by up to 12 dB, so they work like a very broad EQ. Pulling High down 3 to 6 dB is the usual way to take back some of the brightness and hiss that the upward half adds.

OTT vs a normal compressor

How to use OTT, step by step

This order works on a synth, a drum bus or a sound effect. You hear the full effect first, so you know what you're blending in, then dial it back.

1. Put it on the sound and listen at 100%Leave Depth at full for a few seconds to hear what OTT does to this particular sound: which band jumps forward, whether the tails or the noise come up, and what happens to the hits.
2. Fix the band that's too muchIf the top end hisses or spits, pull the high band down a few decibels. If the bass gets boomy, pull the low band down. Doing it here is quicker than chasing it with an EQ afterwards.
3. Balance Upward against DownwardTurn Upward down if the quiet parts have become too loud, or Downward down if the hits have lost their snap. Leave both at 100% if you like what you hear.
4. Lower DepthBring it down until the effect is only just obvious when you switch it off and on. On a whole mix that's often under 25%.
5. Match the level with Out gainBypass the plug-in and set Out gain so both versions sound equally loud. OTT almost always changes the level, and the louder version will win if you skip this.
6. Compare, then decideSwitch it off and on a few times. If you can't say what got better, lower Depth again or take it off.

Starting settings for common sounds

These are starting points, set so you can hear OTT working. Your ears and a level-matched bypass decide where you end up.

Where OTT works, and where it hurts

OTT is at its best in sound design. A synth patch that sounds thin and polite gets thicker and brighter, because its quiet upper harmonics sit in the high band and get turned up towards the level of the body. It often goes at the end of a synth's effects chain, and risers and impacts can take it at full depth, since nobody expects them to sound natural.

On drums it works well at low depth on a bus. The upward half pulls up the room sound and the decay of each hit, so a dry kit sits in the track more easily. Pushed harder, the same thing turns a tight kit into a wash of cymbals.

It hurts most on sounds that are meant to be quiet in places: an acoustic recording with natural dynamics, a vocal with breaths and lip noise between lines, or a whole mix. Upward compression can't tell a soft note you meant from noise you didn't, so it raises both. If you want those sounds denser, a normal compressor or parallel compression usually gets there without lifting the junk.

Common OTT mistakes

Leaving Depth at 100%

Full depth is the effect at its most extreme. It's right for sound design and wrong for almost anything you're mixing, where 10 to 30% usually gives you the density without the hiss.

Judging it louder than the original

OTT changes the level and makes things denser, and a louder, denser version wins every quick comparison. Match the level with Out gain before you decide, or use the A/B in the lab, which matches it for you.

Missing the noise floor

Hiss in a sample or a long reverb tail can sit 40 dB below the music, where nobody notices it. Lift it by 15 dB or more and it becomes part of the sound. Solo the track with OTT on and listen to the gaps; if the noise is there, gate or filter it before OTT, or turn Upward down.

Stacking it without meaning to

The compressors in Serum and Vital both have OTT-style multiband modes, and plenty of synth presets use them. Add the plug-in on the track and another on the bus and the same sound has been through three of them. Check what's already in the chain first.

Using it to fix a mix

OTT makes a muddy mix louder and brighter, which can sound like a fix for a minute. It doesn't change the balance between the parts, though, and the quiet ones come up with everything else. Sort out the levels and the EQ first.

Training your ears to hear OTT

The quiz under the lab covers the controls and matches every round to the original's loudness, then asks which half of the processing is on, or whether Depth is at 25% or 100%. Telling upward from downward is the most useful part to learn. For upward, listen to the gaps between hits; for downward, listen to the hits themselves.

Other OTT-style compressors

Ableton Live's Multiband Dynamics is the device OTT came from, and it shows every setting the plug-in hides: the crossover frequencies, a threshold above and below for each band, and separate ratios for the upward and downward halves. If you use Live and want to move the bands or soften the upward half, load the OTT preset there and adjust it. The compressor effect in the free synth Vital also works on three bands in both directions, and Serum's compressor has a multiband mode built on the same idea.

Questions people ask

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